Originally Posted by
Hypno Toad
Wait... WHAT!?!? Troll much??
No and I resent the implication. Nothing I have said is inflammatory nor off-topic nor extraneous. The math and chemistry I've presented is very germane to the discussion and fairly simple stuff...high school chemistry would cover it.
Originally Posted by
Hypno Toad
I've read that article and it's not convincing. The salient quote is this
In places where cars idle (such as driveways, toll plazas, and intersections), automobile exhaust can be an important contributor. At cold temperatures, auto exhaust (like your breath) contains a visible fog of water droplets, formed when invisible water vapor condenses into droplets. Some of these fog droplets may then freeze when they come in contact with the roadway within a few feet of the tailpipe. More water condenses on the inside surface of the tailpipe and drips out, leaving a thicker but more localized patch of ice directly beneath it.
Some guy on the internet isn't a credible source. Doing a google search, I can find all kinds of pages with almost the same sentence as the first sentence. But I can't find a single journal article on the topic.
And the above certainly doesn't fit with the quote from Wikipedia
At low temperatures (below –18 °C), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface.[4]
At that temperature there are lots of other, more reasonable mechanisms for ice formation.
Originally Posted by
tjspiel
Wait, wait, hold on. I'm not saying that ice formed from car exhaust completely covers the city. Black ice from car exhaust is very localized, - typically at intersections or areas where you have many cars stopped for periods of time or moving slowly. It doesn't take that much water.
And because it's localized and not covering everything, it is more dangerous because it's unexpected.
But it takes more water than you have even from many cars. I do agree with the straight dope guy about one thing, water is going to condense in the tailpipe and dribble out. You can see it from just about any tailpipe of any car. But as he said, the water is going to freeze in a blob and it will likely be outside the tire track which won't cause problems.
But if you condense water in the tailpipe, it's not helping the "freezing car exhaust fog" argument. You would need water vapor to freeze on the road surface in a uniform layer. For a typical stoplight duration and the burn rate of gasoline, you only have 9 g of water to work with. Since you aren't Canadian, I'll convert...that's a bit under 2 teaspoons. If you remove half of the water due to condensation...not a bad estimate but only a guess...you have 5 g (1 tsp) of water to work with. Let's say you have a 3 lane road with 5 cars in each lane. That's 15 cars or 75 ml (15 tsp or about 1/3 cup) to work with.
But goes into the volume that those 15 cars are contained in. That volume is, roughly, 504 cubic meters. 75 g of water (density of 1g/ml) spread throughout that volume would give you 0.15g/cubic meter. If the air isn't saturated it will absorb the water from the tailpipe easily. Minneapolis is currently (like right now) at 75% RH. That means that each cubic meter of air contains 75% of the
possible 1.0g of water it can carry at your current temperature (just about 0°F) or 0.75g/cubic meter. Adding 0.15g/cubic meter of water won't cause water to precipitate out. Perhaps a very tiny amount will but not enough to cause the roads to ice.
Ice may have formed overnight but the cars would just polish that ice resulting in a slicker intersection.